Two lines of experimentation are proposed in this grant directed towards developing a better understanding of both the perceptual process underlying nociception as well as the neuronal mechanisms responsible for the regulation of the response characteristics in ascending somatosensory pathways activated by noxious stimuli. This first group of experiments consists of psychophysical studies directed towards examining the effect of electrical stimulation on the perception of experimentally induced noxious stimuli. These studies will specifically examine the significance of various spatial factors which may be important to the neuronal interactions underlying the perceptual changes induced by this techniques. In addition, the changes in nociception that occur as a consequence of the chronic pain state will also be evaluated. A second group of studies will consist of electrophysiological experiments designed to study the characteristics of a descending pathway originating from the brainstem which has been shown to alter the behavior of spinothalamic neurons to noxious stimuli. Attention will be directed towards examining the nature of the spinal segmental interactions whereby this descending pathway exerts its effect on these neurons. Lastly, studies are also proposed in which the interactions between the trigeminal and spinothalamic systems will be evaluated.